In a conventional technique, e.g., in JP 2009-142854 A, a pair of clamping units as energizing electrodes clamps a workpiece at both ends from vertical direction to electrically heat the workpiece. During the electrical heating, the clamping units move in a separating direction from each other in order to apply a tension to the workpiece, whereby the workpiece can avoid a warp caused by expansion of the material.
JP 2009-142854 A also discloses a structure including multiple pins disposed below the middle of the workpiece, in which the pins support the workpiece under heating to prevent sagging.